...for example gear missions, 8 hour gold & adrenaline buffs, fastest speed buffs, and big idols rewards like the Temple of Dumb? Thanks!

Mouse-over says Speed Runner "Increases the odds of getting shorter duration missions by (10)%"

Sounds like it would Decrease the odds of getting long missions (such as gear missions, etc.)

Which seems undesirable, leaving me to wonder why someone would want to buy into that Talent?

We know of course that the missions work within separate categories. Buckets, as they've been called in posts from the dev team. So there's one mission slot which can be a gear mission, two mission slots which can be buff missions, four mission slots which can be gold missions, etc.

I picture the behind the scenes process working something like this:

1) A particular mission slot is due for a mission to be generated, retrieve list of all missions of that type, remove anything that's ineligible (like if you've already got it in another slot of the same type or if it's on cooldown (recently completed missions have a hidden behind the scenes cooldown before that same mission can be generated again) or whatever else) and then check if it's possible to filter the remaining list to only include "short" missions (side note: the devs define "short" as 4 hours or less)

2a) IF the list can not be filtered (no short missions exist, as is the case for recruitment and gear missions) proceed directly to step 3 with the unfiltered list

2b) IF the list can be filtered roll on a 25% chance (made up number, I don't know what it really is, but at max level Speed Runners that would be boosted 200% to become a 75% chance) to determine if the list will actually be filtered and then proceed to step 3 with either the filtered or unfiltered list according to that result

3) Take the list provided by step 2 and randomly select a mission from it

How beneficial this could be kind of depends on how you're going to be playing the game. If you're going AFK overnight with an autoclicker buying levels for you, you may not care how long the missions are. But when there will be a stretch of play time where you will be actively playing (or at least checking in on the game regularly), having shorter missions come up more frequently could allow you to start and complete missions many times and you would find that longer missions just get in the way of this.

Common EP missions like Storming the Castle, The Magical Forest, and Farmer Brown's Tractor. Which clock in at 5, 10, and 30 minute duration, respectively. As opposed to rare or epic EP missions like Golden Children (2 EP after 2h30m) or Ballet Class (3 EP after 12h). While one of your four EP slots is tied up for two and half hours to earn just 2 EP for each crusader sent out on Golden Children, another EP slot could cycle through those common missions and earn 1 EP at a time at least 6-8 times. (The estimate accounts for 5 minute cooldown before the same slot will generate another mission after completing the previous one.) For a longer one like Ballet Class you could drag that out even longer. While one slot is tied up for 12 hours to earn each of its crusaders 3 EP, another slot could see dozens of 1 EP missions completed if you'll be fairly active through a significant amount of that time.

Where buff missions are concerned... Well, there aren't a lot of them to work with (9 missions total, one of each type in each mission rarity) and the cooldowns all appear to be pretty long. According to the wiki, 6 hours until the slot is free again and 36 hours until a specific mission is free again (common missions at least)... Honestly, because of those cooldowns and the limited selection to randomize, you're pretty much going to cycle through all the different buff missions in turn with or without Speed Runners.

You send out the first two missions. Let's just assume it comes up as 2 of the 3 short ones. And then that's 7 missions for the system to pick from when generating the next missions 6 hours later. Suppose that the next two are both rare missions, so it's about 6 hours for the mission durations and then another 6 hours until the next missions can be picked. It's been 18 hours so the first two missions are still on cooldown and there are 5 missions for the system to pick from. Let's suppose that we're unlucky enough that we don't see any of the epics next. We get the remaining common mission and the remaining uncommon mission. The two mission slots go out of sync here, with one generating the next mission 6 hours from now (at the 24h mark since the start of this whole thing) and the other 12 hours from now (at the 30h mark). All the previous cooldowns are still in effect so the selection is between the remaining 3 missions, which are all the epic buff missions. Of course, any time an epic mission is selected that will tie up that mission slot for the next 24h between the 18h mission duration and the 6h slot cooldown.

EDIT: The shortest idol mission takes 7 hours by the way. The idol mission slot will be unaffected by Speed Runners since there are no idol missions under 4 hours.

There are no idol missions under 4 hours. The shortest one is 7 hours.

But yeah, I don't think it's stated in game anywhere but the official response in forums / AMAs when they've been asked about it is that Speed Runners specifically increases the chances of seeing missions that are 4 hours or shorter.

So if it's time for an EP mission to be generated it looks at a list of EP missions and does "something" (I posted my assumption on the mechanics but I doubt that they'll confirm anything that specific) to ensure that the EP missions at 4 hours or less get a boosted chance to appear.

Or if it's time for a gold mission to be generated it looks at a list of gold missions and ensures that the "short" ones there get a boosted chance.

Likewise for each other category of mission. If it's time for a mission of type X to be generated it looks at a list of available missions of type X and provides a boost for any missions on that list which are 4 hours or shorter. If it's time for an idol mission to be generated, it will look at the list and see no missions of that length so there will be no boost to apply.